The radioactive isotope 241 Am is one of the most commonly used alpha emitters in civilian applications. As such, the possibility of deliberate contamination of the environment with this isotope is not negligible. Detection of a non-shielded 241 Am source is possible by measuring its characteristic 60 keV gamma ray. However, this gamma ray is easily absorbed when the source is shielded. Therefore, the detection of a concealed 241 Am source should rely on monitoring the source's low intensity gamma rays in the energy ranges of 300 to 400 and 600 to 750 keV, and on the detection of neutrons generated inside the source active volume via the (a,n) reaction. In this work, the gamma spectra and neutron flux associated with an 241 Am source were measured and calculated for several shielding configurations. Based on these results, the minimum time needed for the detection of a concealed 241 Am source was estimated and a concept for passive detection and identification of this source, as well as other alpha emitters, was investigated. This concept was then verified by a set of measurements conducted on an advanced spectroscopic portal located in Haifa harbour.
This work introduces a new approach for designing an Advanced Spectroscopic Portal (ASP). The new design is based on multiple, small (7.62 cm x 7.62 cm) and commercially available NaI(TI) scintillation detectors. A comparison to another approach, which employs very large detectors, is presented in the aspects of source localization, signal to noise ratio (SNR), and identification and detection performances. The advantages of the introduced approach are discussed. In addition, a mathematical model applied within Rotem's ASP, Radiation Material Detection System (RMDS), for improving the Minimum Detectable Level (MDL) is described. This algorithm shows explicitly the advantage in usage of multiple detector modules over few larger having the same scintillator type and volume.
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